1. Field of the Invention
This invention relates to ink jet printing and more particularly to a thermal ink printhead having increased frequency response enabled by ink channel geometry providing faster refill times.
2. Description of the Prior Art
Thermal ink jet printing is a type of drop-on-demand ink jet printing which uses selectively applied thermal energy to expel ink droplets by producing momentary vapor bubbles in ink-filled channels of a printhead. A thermal energy generator, usually a resistor, is located in each of a plurality of channels near the nozzles at one end thereof. The other ends of the channels are in communication with a common manifold or reservoir which contains a source of ink.
U.S. Pat. No. 4,463,359 to Ayata et al, discloses one or more ink filled channels which are replenished by capillary action. A meniscus is formed at each nozzle to prevent ink from weeping therefrom. A resistor or heater is located in each channel upstream from the nozzles. Current pulses representative of data signals are applied to the resistors to momentarily vaporize the ink in contact therewith and form a bubble for each current pulse. Ink droplets are expelled from each nozzle by the growth and collapse of the bubbles. Current pulses are shaped to prevent the meniscus from breaking up and receding too far into the channels, after each droplet is expelled. Various embodiments of linear arrays of thermal ink jet device are shown, such as those having staggered linear arrays attached to the top and bottom of a heat sinking substrate and those having different colored inks for multiple colored printing.
U.S. Reissue Pat. RE No. 32,572 to Hawkins et al, discloses several fabricating processes for ink jet printheads, each printhead being composed of two parts aligned and bonded together. Many printheads can be simultaneosly made by producing a plurality of sets of heating element arrays with their addressing electrodes on, for example, a silicon wafer and by placing alignment marks thereon at predetermined locations. A corresponding plurality of sets of channels and associated manifolds are produced in a second silicon wafer and in one embodiment alignment openings are etched thereon at predetermined locations. The two wafers are aligned via the alignment openings and alignment marks and then bonded together and diced into many separate printheads.
U.S. Pat. No. 4,638,337 to Torpey et al discloses an improved thermal ink jet printhead similar to that of Hawkins et al, but has each of its heating elements located in a recess. Recess walls containing the heating elements located in a recesss. Recess walls containing the heating elements prevent the lateral movement of the bubbles through the nozzle and therefore the sudden release of vaporized ink to the atmosphere, known as blow-out, which causes ingestion of air and interrupts the printhead operation whenever this event occurs. In this patent, a thick film organic structure, such as Riston.RTM., is interposed between the heater plate and the channel plate. The purpose of this layer is to have recesses formed therein directly above each heating element to contain the bubbles generated by the heating element, enabling an increase in droplet velocity without the occurrence of vapor blow-out.
Pending U.S. patent application Ser. No. 115,271, to Hawkins, entitled "Ink Jet Printhead", filed Nov. 2, 1987, now U.S. Pat. No. 4,774,530, discloses an improved printhead which comprises an upper and lower substrate that are mated and bonded together with a thick insulative layer sandwiched therebetween. One surface of the upper substrate has etched therein one or more grooves and a recess, which when mated with the lower substrate, will serve as capillary filled ink channels and ink supplying manifold, respectively. Recesses are patterned in the thick layer to expose the heating elements to the ink, thus placing them in a pit and to provide a flow path for the ink from the manifold to the channels by enabling the ink to flow around the closed ends of the channels, thereby eliminating the fabrication steps required to open the groove closed ends to the manifold recess so that the printhead fabrication process is simplified.
The above disclosed thermal ink jet printheads have a relatively long channel through which ink is supplied from the reservoir to the nozzle. The heatig elements which produce the bubbles are placed in pits in the channel a predetermined distance upstream from the nozzle openings. The pits prevent bubble blow-out, thus avoiding printhead failure. Unfortunately, the maximum frequency of such printheads is around 3 KHz. The operating frequency is governed by the channel refill time, and the channel offers the maximum resistance to flow. The present invention increases the droplet generation rate or operating frequency by minimizing the channel resistance to flow.